1. Field of the Invention
The present invention relates to a rubber composition and more particularly to a rubber composition which will find best use in application areas such as the tread of passenger car tires where high abrasion resistance and resilience are required.
2. Description of the Prior Art
Recently, there has arisen a demand for fuel saving of passenger cars through improvement of tires. To meet this demand, researches are being carried out on the rubber material in varied aspects.
The contribution to fuel saving by passenger car tires is realized by reducing the rolling resistance of the tire tread which is encountered during running. It is said that the reduction of rolling resistance is effectively accomplished by imparting to the rubber material the dynamic performance such as high resilience and low heat build-up which reduces the loss of driving energy. It is also said that dynamic properties of rubber can be improved by incorporating rubber with a specific type of carbon black, e.g., so-called soft black of coarse particles or a carbon black having such a broad particle (and aggregate) size distribution that the tinting strength at a given particle size is reduced to a great extent. Such a carbon black, however, tends to be inferior in rubber reinforcement; that is, a rubber composition compounded with it is poor in abrasion resistance which is one of the most important properties required for tire treads.
The present inventors have discovered that these contradictory requirements can be met by a rubber composition compounded with a carbon black having the following properties. The particle diameter is in the range of SAF (super abrasion furnace) and ISAF (intermediate super abrasion furnace). The structure is comparatively low. The true specific gravity is in a specific range (.ltoreq.-0.0006.times.N.sub.2 SA+1.8379) which is much lower than that of known carbon blacks having the corresponding particle diameter. Owing to this finding, the present inventors were granted Japanese patent No. 1,120,426 (Japanese patent publication No. 34149/1978). This Japanese patent, however, substantially lacks the practical value. According to it, the carbon black which imparts both high abrasion resistance and low heat build-up to the rubber composition is required to have its characteristic values in narrow ranges. That is, the specific surface area measured by the nitrogen adsorption method (hereinafter abbreviated as N.sub.2 SA) should be 94 to 145 m.sup.2 /g, and the absorption of dibutyl phthalate (hereinafter abbreviated as DBP) should be 89 to 107 ml/100 g.